MSU Extension Fruit & Nuts

San José scale was brought into California from China in the early 1870s and quickly became a serious pest of apples, peaches and plums. It spread rapidly across the U.S., becoming a pest of tree fruits in nearly all production regions. The impact of San José scale declined substantially with the introduction of modern insecticides into control programs in the 1950s. Dormant oil sprays targeting overwintering scale and in-season sprays targeting crawlers kept this pest in check for many decades. However, within the past few years San José scale has emerged as a serious pest of Michigan apples and sweet cherries.

Despite targeting San José scale with recommended control measures, apple growers have sustained severe economic losses due to the build-up of populations in their orchards. Losses primarily result from the presence of scale insects themselves and the conspicuous red spot caused by their presence, both of which render apple fruit unmarketable. A photo of infested apples harvested from an orchard in Michigan’s Fruit Ridge area in 2018 is shown in Photo 1.

Economic impacts in cherry result from feeding on tree tissue rather than direct damage to the fruit, although fruit feeding can occur in high populations. Fortunately, we have not observed San José scale damage to sweet cherry fruit in Michigan. San José scale infestation in sweet cherry causes tree decline, and in serious cases, tree death. The recent and relatively widespread build-up of San José scale populations in sweet cherries has resulted in substantial losses of fruiting capacity.

There are several possible reasons for the surge in San José scale populations in Michigan commercial orchards. There has been a general decline in the use of delayed-dormant oil sprays, in part because the cost of these treatments has increased. New regulations governing insecticide registrations have removed or limited the use of some insecticides most effective against San José scale. Many of these were broad-spectrum materials that controlled a key pest, such as codling moth, but incidentally also kept San José scale in check. Growers and consultants typically do not inspect the woody parts of the tree for infestation, but only notice San José scale in their orchard when they are present on fruit. In these cases, San José scale may go unnoticed and unchallenged, allowing populations to build to levels that are more difficult to control with chemical options.

It is also possible that San José scale has developed tolerance to the most commonly used compounds, such as Lorsban (chlorpyriphos). Indeed, documented resistance of San José scale to lime sulfur was the first case of a tree fruit insect becoming resistant to an insecticide. Additionally, many of the newer materials are not as directly lethal as the older chemistries and may not be providing the near complete control that was achieved in the past, increasing concerns about the development of resistance. Lastly, some of the newer chemistries have different modes of action that can complicate determining the appropriate time to make applications that will successfully control San José scale.

San José scale overwinters in the black cap stage and resumes development in spring when temperatures exceed 51 degrees Fahrenheit. Around apple bloom, mature females and males emerge and mate. Targeting overwintering scales as they begin to develop in the spring, well before adults emerge, is a worthwhile strategy for managing this pest. This early season timing is generally referred to as the delayed dormant period in apple, which covers as close to green tip as possible and up until the pink stage. Early season San José scale sprays in sweet cherry are also applied before green tissue is present, particularly if Lorsban is included in the tank as this material is phytotoxic to sweet cherry foliage. Unfortunately, targeting San José scale during this early timing can be very difficult to achieve as cold spring temperatures can also make oil applications risky. While good coverage is important for all spray applications, this is especially true when using oil to target overwintering San José scale that are on and under bark scales and nestled behind buds in spurs.

Horticultural oil applied at the delayed dormant stage will control San José scale by smothering the overwintering scale. This application would also provide activity against overwintering European red mites which were in high populations in some Michigan apple orchards last season. A common way to use dormant oil in apple is 2 gallons per 100 gallons water per acre at green tip with copper. Some growers will break up their oil applications into two sprays: once at green tip with copper and another at 1 gallon per 100 at tight cluster with an insecticide targeting scale and rosy apple aphid. Rates need to be reduced as the oil is applied closer to pink. Spur and bud damage is a high risk at pink and should be down to no more than a gallon of oil per 100. Late applications will offer the best help to control scale and mites and adding an insecticide will help control rosy aphids.

In sweet cherries, growers have used similar rates: 2 gallons of oil per 100 gallons of water per acre and included Lorsban in the tank for a dormant-prebloom application targeting San José scale. Although an early application of oil could give growers a jump on San José scale management, this material should be used with caution.

Keep in mind that for all tree fruits, oil and green tissue are not very compatible. Phytotoxic damage to buds, blossoms and fruitlets is possible, thus care must be taken to avoid weather extremes. Do not use oil sprays 48 hours before and after a frost event. Avoid using oils in very hot (over 85 F) and humid conditions. Do not apply sulfur or Captan within several days of oil applications or phytotoxicity will occur.

The three most effective insecticides for controlling scale early in the season are Lorsban (chlorpyriphos), Esteem (pyriproxyfen) and Centaur (buprofezin). Foliar applications of Lorsban may be used dormant/delayed-dormant (pre-bloom) in pome and stone fruits for scale, leafroller and aphid control, either alone or in combination with oil. Keep in mind, if chlorpyrifos is used at this time, it cannot be used in a post bloom trunk application.

Esteem is an insect growth regulator (IGR) insecticide that acts by suppressing development of the eggs. Applying 5 ounces per acre of Esteem between green tip and pink in apples and pre-bloom in sweet cherries with oil to serve as a penetrant provides excellent control of overwintering San José scale. Centaur is also an IGR insecticide that acts on insect nymph stages by inhibiting chitin biosyn­thesis, thus interfering with insect molting. A single application of Centaur at 34.5 ounces per acre is an excellent option and the addition of 1 quart of oil or a penetrant surfactant (0.25 percent) has been shown to increase control. Additional information and recommendations can be found in Michigan State University Extension’s 2019 Fruit Management Guide (E-154).

Follow all label guidelines. Dormant oil needs to be applied with care and attention to the weather conditions before and after the application.